Amino-functionalized Ti-metal-organic framework decorated BiOI sphere for simultaneous elimination of Cr(VI) and tetracycline

Abstract

A subtle flower-like MIL-125-NH₂@BiOI was fabricated by a facile solvothermal method for simultaneously eliminating Cr(VI)/tetracycline mixed pollutants under visible light. The strong interaction between amino in MIL-125-NH₂ and Bi³⁺ of BiOI promotes the formation of this unique inlaid structure and enables the favorable contact between MIL-125-NH₂ and BiOI, thus accelerating the transfer of charge carriers. Remarkably, MIL-125-NH₂@BiOI displays a superior activity compared with that of two monomers for the photocatalytic reduction of Cr(VI) and degradation of tetracycline. More significantly, the photocatalytic efficiency can be further boosted in the coexistence of Cr(VI) and tetracycline, which is 1.8 and 1.6 times that of single Cr(VI) and tetracycline, respectively. The synergistic effect between Cr(VI) reduction and tetracycline oxidative degradation can further facilitate the separation of photo-induced electrons and holes, resulting in the improved efficiencies in the Cr(VI)/tetracycline coexistent environment. This work sheds light on that MOF-based photocatalysts possess huge potential for practical environmental remediation.

Introduction

Heavy metal ions and antibiotics in wastewater have always been a serious threat to human beings [1], [2]. Chromium-based materials are widely used in many industrial fields, such as chrome plating, pigment manufacturing, electroplating, leather tanning, textile dye manufacturing and wood preservation [3], [4], resulting in intensive production and discharge of Cr(VI). Therefore, Cr(VI) has become a major component of heavy metals in wastewater and is listed as one of the world's ten most harmful substances due to the huge toxicity [5], [6]. Considering the less toxicity and easy separation of Cr(III) [7], reducing Cr(VI) to Cr(III) would be a feasible way to relieve the pollution of Cr(VI) [8]. On the other hand, tetracycline is generally applied to prevent and control infectious diseases in animal husbandry and fisheries [9], [10]. However, the widespread and excessive use of tetracycline inevitably brings about their abundant existence in water, eventually leading to severe water pollution [11]. To address the above environmental issues, different treatment technologies were employed [12], in which photocatalysis stands out owing to the high efficiency, green, sustainability and no secondary pollution [13]. More encouragingly, the reduction and oxidation can be performed concurrently by photocatalysis, giving the possibility to simultaneously eliminate the mixed pollutants with high efficiency. Thus, several photocatalysts based on the simultaneous removal of Cr(VI) and tetracycline have been designed and reported, examples like AgI/BiVO₄ [14], BiVO₄/FeVO₄@rGO [11] and Co₃O₄/g-C₃N₄ [1]. Nevertheless, significant improvement in catalysts’ photocatalytic efficacies is still required toward practical applications.

Benefiting from the enviable physicochemical property, metal-organic frameworks (MOFs), built via the self-assembly of metal ions or clusters with organic linkers, have attracted extensive interest in various fields [15], [16], [17], [18], [19]. As for photocatalysis, MOFs can not only serve as a semiconductor-like material to promote the generation and transmission of charge carriers, but also provide interaction to combine with others based on the elaborate free functional groups like carboxyl and amino [2], [20]. Thus, the delicate integration between MOFs and other semiconductors to construct heterostructure may offer the opportunity to optimize the photocatalytic efficiencies of MOFs. Bismuth oxyhalides (BiOX, X  = Cl, Br or I), as one of the most classical two-dimensional semiconductors, possess unique optical and electronic features, which render them promising candidates for photocatalysis [21], [22], [23], [24]. In comparison to other two bismuth oxyhalides, BiOI equips the minimum bandgap (∼1.8 eV) [25], [26], making it more advantageous for the absorption of visible light and the production of electrons and holes. Yet, the photocatalytic activity of BiOI usually suffers from the high recombination rate of photo-generated electrons and holes and its low surface area. Accordingly, integration of BiOI with MOFs is a rational and hopeful strategy to conquer the defects of BiOI. Although a few BiOI/MOF composites (e. g. BiOI/MIL-53(Fe) [27] and BiOI/NU-1000 [28]) have been developed, the interaction between functional groups of MOFs and BiOI is rarely studied.

Amino-functionalization for MIL-125 (MIL-125-NH₂) not only extends the light absorption to the visible-light region, but also enhances the interaction with BiOI to some extent. Thus in here, MIL-125-NH₂ was delicately inlaid into three-dimensional (3D) BiOI by a facile solvothermal method for the simultaneous reduction of Cr(VI) and degradation of tetracycline. The strong interaction between amino in MIL-125-NH₂ and Bi³⁺ of BiOI promotes the formation of this structure and ensures the tight assembly of MIL-125-NH₂ and BiOI. By virtue of the favorable contact between BiOI and MIL-125-NH₂, the photocatalytic performances of MIL-125-NH₂@BiOI are significantly enhanced compared with that of pristine BiOI and MIL-125-NH₂ for Cr(VI) reduction and tetracycline degradation. More impressively, the photocatalytic efficiencies are further boosted when Cr(VI) and tetracycline were mixed. Beyond these, to achieve the facile recovery of photocatalysts, the MIL-125-NH₂@BiOI bacterial cellulose aerogel was processed for the synchronous removal of Cr(VI) and tetracycline as well, further improving its practicability.